Project Summary Abstract Stockpiled during World War II, Phosgene Oxime (CX; dichloroform oxime) is a potent chemical weapon that poses a threat of exposure; both, alone and with other chemical agents. It is an urticant or nettle agent grouped with vesicating agents due to similar damaging properties; although, it causes more severe damage than other vesicants due to its highly reactive nature. Even though it is the most notorious vesicant with special military and terrorist interests, it is one of the least studied chemical warfare agents with no specific antidote available. Information on its effect on human dermal tissue and absorption is limited, and its mechanism of action is unknown. To overcome these limitations, our completed and ongoing studies are directed towards the development of a relevant cutaneous CX exposure mouse injury model to elucidate the mechanisms of skin damage by CX and examine systemic toxic effects of CX to identify novel therapeutic targets. The results from our completed studies demonstrate that mast cells could be important players in CX-induced toxicity. The toxicity response and skin urticaria from CX resembles anaphylactic reaction and urticaria from allergic reactions, which involves an inflammatory response mainly due to mast cell activation. Data from our published and completed studies in SKH-1 hairless mice show that CX cutaneous exposure causes mast cell degranulation and release of mediators including histamine and tryptase, pro-inflammatory cytokines, and an inflammatory response in the skin tissue associated with edema, erythema, necrosis, urticaria and blanching. CX cutaneous exposure in mice also caused vasculature dilation and blood congestion in multiple organs resulting in systemic toxicity, and decrease in breath and heart rate, temperature drop and mortality. These symptoms were similar to anaphylaxis (potentially lethal multisystem allergic reaction with acute respiratory and cardiovascular compromise leading to unconsciousness, shock and mortality), which is a result of sudden systemic release of mediators from mast cells. Based on our published and recently obtained data under the current R21 grant, we hypothesize that mast cell activation and associated release of mediators are the major events following CX cutaneous exposure which cause inflammatory pathway activation as well as lethal allergic reaction, and that these would be novel targets for therapeutic intervention to mitigate CX-induced skin morbidity and mortality. To test this hypothesis, the specific aims proposed are: 1. To further establish mast cells as key players and molecular targets in CX toxicity by employing mast cell deficient mice; and 2. To test the efficacy of FDA approved therapies that can counteract CX-induced morbidity and mortality from cutaneous exposure in mice, mainly by targeting mast cell activation and release of histamine. We believe that outcomes from above aims will help establish that mast cell activation and mediators like histamine are novel targets for therapeutic intervention, and identify an FDA approved targeted therapeutic strategy that can target these to counteract toxicity from CX cutaneous exposure.